This invention relates to a radio transceiver and, more particularly, to a radio transceiver for use in a digital cordless telephone system called a PHP (Personal Handy Phone) system which has been proposed as a second-generation cordless telephone.
In a cordless telephone system which is one of mobile communication systems, voice or the like is transmitted as an analog signal over a radiowave transmission section between a coupling device (a master set) and a cordless telephone set (a slave set); at present, a digital type cordless telephone system which digitizes the radiowave transmission section to further enhance the performance of the system is being studied as a second-generation cordless telephone system.
There has been proposed a TDMA-TDD system as one of transmission systems for digitizing the radiowave transmission section.
The TDMA-TDD system is a communication system which utilizes, for transmission and reception of signals between a master and a slave set, a TDD (Time Division Duplex) system in which the master set and the slave set are allowed to use the same carrier frequency for duplex use of one channel on a time-division basis and in which one master set and a plurality of slave sets are controlled by a TDMA (Time Division Multiple Access) system.
The above-mentioned TDD transmission system is also called a TCM (Time Compression Multiplexing transmission) system and may sometimes be called a Ping-Pong transmission system after its time control method.
Generally speaking, in case of a mobile communication, a distance between a mobile station and a base station changes every moment as the mobile station moves and their receiving electric field intensities vary with the position of the mobile station, and hence their received inputs undergo substantial changes accordingly. In the case of a cordless telephone, a slave set (a mobile set) is sometimes brought into close proximity with a master set (a controller), so that their received input levels exceed respective normal levels. In this instance, the existing analog cordless telephone set suffers only degradation of the regenerated or reproduced speech quality, but in the case of the digital cordless telephone of the TDMA-TDD system there is a fear that the detection of a synchronization signal, such as burst synchronization or TDD synchronization, is disturbed, making reception impossible. For example, the received input level is set such that the receiving circuit operates normally in the range of the received critical level (the minimum level at which reception is possible) of 10 dB/.mu.V to the maximum level of 70 dB/.mu.V, but in the case where the slave set is brought close to the master set and the maximum level exceeds 70 dB/.mu.V and enters into the range of 100 to 120 dB/.mu.V, the reproduced speech quality is seriously deteriorated, besides no signals can be received.
To solve this problem, the slave set and the master set are designed as described below.
FIG. 5 is a circuit diagram showing an example of the principal part of a conventional transceiver. In FIG. 5 reference numeral 1 denotes a duplexer, 2 a receiver (Rx), 3 a transmitter (Tx), 4 a switch controller for the duplexer 1, 5 a controller and 8 a variable attenuator.
The controller 5 controls the operation of the entire transceiver circuit and always detects the received input level by its internal logic IC and, when the received input level exceeds a given maximum level and the input level exceeds a normal level, the controller applies a control signal to the variable attenuator 8 to increase an attenuation value, putting the input level of the receiver 2 into the given range.
That is, the variable attenuator 8 is provided at the input side of the receiver 2 and its attenuation value is controlled automatically or semi-automatically to prevent the reproduced speech quality from being impaired when the input is excessive.
The variable attenuator 8 is formed as an electronic variable attenuator using an FET or PIN diode and is very effective for an over input level, but when the received input level is so low as to reach the critical level, that is, when the attenuation value by the variable attenuator 8 is minimum, it provides an attenuation (an insertion loss) of about 1 to 2 dB, causing an increase in the noise figure of the receiver. Consequently, the receiving sensitivity decreases in correspondence to the insertion loss so that the synchronizing operation becomes unstable, reducing the service area.